The game doesn't treat the mouse's default horizontal range as 0-639 for all of the video modes that it supports, but that is just wrong with any MS-compatible mouse driver that I've seen, and in fact the misplaced clicks occur when loading real MS Mouse drivers. Also, even though the game allows you to select Tandy with mouse, it intentionally disables the mouse for Tandy and you can only use keyboard.

I made a program that fixes both problems: mouse can be used with Tandy and horizontal range is always handled as 0-639. It's a fix for the game, not a workaround for any issue in DOSBox; however, it is specific to v2.3 -- do you have that version?

Can't read it off the disk the label fell off ages ago and don't have the box any more.... I "think" it might be, when I start the game it gives the line "020888 iv2.3" at the top of the screen.

Put the program in the attached archive in the game directory and run it to start the game. If there is no effect then the game executable is different despite having the same version number.

I'll warn you now that using the mouse with Tandy graphics will not work correctly in DOSBox 0.74, and it will only work in SVN with the get_pixel_tandy16 patch that is not committed. DOSBox's internal mouse driver uses GetPixel() to save the mouse pointer's background, and without support for the video mode you'll be painting black pixels as you move the pointer.